The present invention relates to the production of metal oxides, such as ceramic materials and high temperature superconductors. The preparation of ceramic materials typically involves grinding mixtures of ceramic oxides often including silicon dioxide and sintering the resulting mixture at high temperatures, e.g., 900.degree. to 1200.degree. C.
High temperature superconductors are prepared in a similar manner, without silicon dioxide. Superconductors do present a more complicated problem than other ceramic materials in that one must not only obtain an integrated mass as in the case of a ceramic object, but also a mass which is superconductive at temperatures above 23K, preferably above 77K, the temperature of inexpensive liquid nitrogen. There is less predictability in whether or not a given method of preparation will yield such a superconductor than there is in predicting whether or not one will obtain an integral mass.
It is well-known that the orthorhombic phase of perovskite structure for Ln.sub.1.0 Ba.sub.2.0 Cu.sub.3.0 O.sub.(7-x) (sometimes referred to as "1-2-3") is a high T.sub.c superconductor material. Ln is preferably yttrium (Y), but can also be lanthanum (La), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). It is also well-known that the orthorhombic phase can be easily obtained from the tetragonal phase, by oxidation in oxygen and by annealing at about 500.degree. C. Early routes to this material involved grinding the oxide or carbonate phase of yttrium, barium and copper, then sintering and ultimately- annealing in oxygen.
It was recognized that useful applications for this material would require thin film geometries not easily obtained by the above mentioned processes. Therefore, numerous processes were developed for processing this ceramic material into more useful shapes such as films and fibers. Thin film geometries are especially difficult.
Several high energy vapor routes were developed for formation of films. These processes, though useful, do not lend themselves to simple coating processing such as dip coating, flow coating or spin coating It was therefore desirable to develop solution coating procedures. One was developed by dissolving nitrate salts of yttrium, barium and copper in hot water. Upon cooling, the material separated into phases, so the process must be carried out under the difficult conditions of hot solvents. The so-called metallorganic or sol/gel processes were also developed. The sol/gel precursors fall into three classes: metal salts of fatty acid esters, metal alkoxide or metal .beta.-diketone complexes dissolved in organic solvents. These frequently suffer from low solubility so that concentrated solution of the mixed salts is difficult to obtain. Thus, aging or refluxing was sometimes required.